The outstanding strength to weight characteristic of composite materials, for example, carbon fibre reinforced resins, has led the aircraft industry to the use of such materials in airframe structural components. However, use of carbon fibre reinforced resins has been limited by the difficulty of mechanically fastening such materials to one another or to structural components of the airframe employing other materials. Conventional mechanical fasteners are unsatisfactory for several reasons. One characteristic of composite materials is that the material adjacent the periphery of an aperture therein is subject to shattering or cracking when relatively high unit pressure is brought to bear thereagainst. Conventional metal rivets often exhibit metal flow incident to the head upsetting operation that induces high unit pressure on the periphery of the rivet hole. Fracture of the composite structure under or adjacent the rivet head may not be evident or even immediate. However, subjection of an airframe to the vibratory environment encountered during normal flight conditions or severe loading as is experienced in inclement weather conditions may result in failure of the composite structure.
Metal screws are generally unacceptable for fastening composite materials since unit stress in the thread form is relatively high.
Conventional metal bolts with nuts thereon are subject to a weight penalty.
To solve the aforesaid problem, the aircraft industry is currently utilizing adhesives to join composite components. However, adhesively bonded joints generally fail prior to failure of the structural components, which is contrary to fundamental design criteria in the aircraft industry.
While attempts to solve the aforesaid problem have been made using plastic rivets, such rivets heretofore known and used do not exhibit sufficient shear strength to be suitable for aircraft applications.
For example, Lyon application Ser. No. 342,176 entitled Blind Rivet, and assigned to the assignee of the instant invention, teaches forming a plastic rivet by the application of heat to first soften and thereafter fully polymerize a "B" stage thermoset resin. The rivet comprises a hollow, headed body of "B" stage thermoset resin that is supported by a headed mandrel. After heating of the rivet body the mandrel is pulled relative to a workpiece thereby forming a head on the blind side of the workpiece. The "B" stage thermoset resin thereafter fully polymerizes to the thermoset condition. Such rivets exhibit shear strength in the range of 15,000 p.s.i.
It is evident, however, that fastening of composite components that exhibit a shear strength of 50,000 p.s.i. with relatively low strength plastic rivets does not produce an assembly suitable for aircraft structural applications. Thus, a need exists for a rivet that exhibits physical characteristics similar to modern composite materials.